In regard to a method for manufacturing a valve umbrella portion of a hollow engine valve, the inventor of the present application made the invention of Patent Document 1 to be described below. Its outline will be given as follows: The valve umbrella portion of the hollow engine valve, particularly in an exhaust valve, is exposed to high temperatures. For the valve umbrella portion, therefore, use has been made of materials showing excellent properties including heat resistance, such as heat resisting steels based on manganese, nickel, chromium, etc.
These materials have the advantage of being highly resistant to heat, but also has the disadvantage of being poor in plastic workability. That is, it is difficult to forge them into the finished form of the valve umbrella portion, and a hollow has to be provided in the hollow engine valve, thus making working even more difficult. Thus, in forming the valve umbrella portion by forging such a material, it has been common practice to raise the temperature of the material to a value equal to or higher than its recrystallization temperature, and carry out working by hot forging.
With hot forging, however, the results are necessarily poor such that working accuracy declines owing to the problem of metal expansion or the like, and that the texture of the surface of the product is inferior to that in cold forging.
Under these circumstances, the inventor of the present application sought a method for forming a valve umbrella portion by cold forging, not hot forging, with the use of a material with high heat resistance as mentioned above. By trial and error, the inventor worked out a method comprising producing, first, a valve umbrella portion semifinished product in which the maximum outer diameter of an expanded-diameter section agrees with the maximum outer diameter of the valve umbrella portion as a finished product, and which has a cylindrical hollow with a bottomed lower end having the same inner diameter as the maximum inner diameter of a valve umbrella portion hollow of the valve umbrella portion as the finished product; and then gradually drawing the semifinished product in a plurality of stages by cold forging, with the upper part of the expanded-diameter section and the body being targeted, to prepare the valve umbrella portion as the finished product. The inventor filed an application for this method, and this application was granted a patent right (Patent Document 1 to be described below).
As the materials for the valve umbrella portion, the following three types were named:
NCF 47W (nickel-based steel)SUH 35 (austenitic manganese-based steel)Inconel 751 (nickel-based steel)
The inventor of the present application repeated, many times, confirmation experiments on the above materials even after acquisition of the patent right of the Patent Document 1. As a result, NCF 47W and Inconel 751 were confirmed to obtain a valve umbrella portion as a finished product, without any problem, by the method of the Patent Document 1. Materials with a high carbon content included in JIS 4311 heat resisting steels (SUH 35 also included therein) were found to show trouble, such as cracking or deformation, slightly more frequently than NCF 47W and Inconel 751, when all stages of necking (drawing) were performed by cold forging.
In recent years, demand has tended to surge for the low fuel consumption of vehicles, and there has been a tendency to demand that all vehicle components be compact and lightweight. With such trends, hollow engine valves have attracted attention, particularly, because of the weight reduction of members which repeat rapid reciprocating motions within an engine. The tendency is growing toward a keen desire for highly accurate forging using various materials including materials with unsatisfactory cold forgeability.